1. Field of the Invention
This invention relates to an alkaline aqueous detergent for removing lubricants from the surface of tin-plate, particularly from tin cans, having a ph of 9 to 13 and containing an alkaline earth metal salt.
2. Statement of the Related Art
Tin cans have traditionally been manufactured in three pieces, consisting of a can cylinder, a can lid, and a can bottom. There is presently a trend toward manufacturing tin cans in only two pieces, with an integral cylinder and bottom.
These two-piece cans are manufactured by stamping tin plate into a circular form, pressing it into a cup shape, and then putting it through a process called drawing and ironing (referred to below as the "DI process"), in which it is passed through several stages of dies to form the can cylinder and bottom in one body. In doing so, a lubricant consisting of mineral oil, animal or vegetable oil, surface active agents, oil property enhancers, extreme-pressure additives, etc., is used to protect the surfaces of the dies and the can and to make the DI process function easily. The DI-process tin-plated can is ordinarily degreased and then chemically treated, after which, if desired, it is painted. In the degreasing, an alkaline degreaser is generally used. If a degreasing detergent with a strong alkaline builder is used to thoroughly remove the above-mentioned lubricant, a sufficient degreasing detergent effect may be obtained, however, one also invites dissolution of the tin on the can surface, the tin-steel alloy, or the steel substrate itself. This not only damages the appearance of the can surface, but can also result in poor corrosion resistance even when subsequent chemical treatment and painting are performed, so that it becomes useless as a container for foods, drinks, etc.
Various cleaning compositions for tin-plate or other metal surfaces are known in the art, including the following, listed in numerical order.
U.S. Pat. No. 2,037,566--Durgin discloses a cleaner composition for tin comprising at least one of tri-sodium phosphate, sodium carbonate, sodium metasilicate, borax, or soap powder in combination with an alkali metal perborate as well as an alkaline earth metal salt and/or an alkali metal silicate.
U.S. Pat. No. 2,142,870--Hall, et al., discloses a composition cleaner for tinned surfaces comprising tri-sodium phosphate and sodium carbonate or sodium sesquicarbonate. Sodium bicarbonate is excluded.
U.S. Pat. No. 3,007,817--Cavanagh, et al., discloses cold cleaning a metal surface prior to a phosphate coating using an alkaline cleaning composition comprising alkali metal orthophosphates and borates, sodium being preferred. Sodium nitrite and an octylphenoxy ethanol surfactant may also be present in the cleaner.
U.S. Pat. No. 3,888,783--Rodzewich and its divisional, U.S. Pat. No. 3,975,215 disclose a cleaner composition for tin-plated ferrous metal cans comprising an alkali metal metasilicate, an alkali metal condensed phosphate, borax, and optional surfactants and wetting agents, preferably nonionic
U.S. Pat. No 4,259,199--Wee, et al., discloses an alkaline dishwasher detergent composition comprising a sodium or potassium tripolyphosphate, sodium or potassium carbonate to raise the reserve alkalinity, sodium or potassium silicates, a chlorine source such as sodium dichlorocyanurate dihydrate, a nonionic surfactant, and other minor ingredients.
U.S. Pat. No. 4,265,780--Kimura, et al., discloses an alkaline cleaner composition for tin cans comprising a myoinositol ester, an alkaline builder which may be at least one of sodium secondary phosphate, sodium tertiary phosphate, sodium carbonate (soda ash), sodium bicarbonate, and the like, and a surfactant.
U.S. Pat. No. 4,490,181--McCready discloses an alkaline cleaner composition for tin cans having a pH of 11 to 13 and comprising an alkaline component which is at least one of alkali metal hydroxides, carbonates, and silicates and ammonium hydroxides and carbonates with an etching inhibitor which is a substituted benzene, a quinone, or a substituted quinone.
Canadian Pat. No 563,357--Arnold, et al., discloses a non-ferrous metal cleaner composition preferably having a pH of 9 to 11 comprising soda ash, sodium tripolyphosphate, tri- and mono- sodium phosphate, sodium nitrite, and a nonionic surfactant, among others.
Published Japanese Pat. application No. 57-15,670 discloses a nitrite as one ingredient in an alkaline degreasing composition for metal surfaces. The nitrite is identified as an oxidant, the group of oxidants including a bromate, chlorate, iodate, chromate, vanadate, permanganate, etc.
Another kind of alkaline degreasing and cleaning agent comprises a combination of an alkaline ingredient and a surface active agent. To increase the cleaning power, a suitable combination of high-temperature, high-concentration, or strongly alkaline treatments is used: A problem is created, however, in that excessive dissolution of the tin and the iron (or steel) substrate is caused along with the increased cleaning power. This is particularly important in view of the situation of recent years, in which the price of tin has risen and, as a result, the quantity (thickness) of plated tin has been reduced. Therefore, the availability of a cleaning agent which can remove the abovementioned oils and oxide film satisfactorily without causing excessive dissolution of the tin and the iron (or steel) substrate has become urgently required.
Various kinds of cleaning agents have been proposed with the purpose of suppressing the excessive dissolution of the tin and the iron (or steel) substrate. For example, a cleaning agent has been proposed which adds a tannic acid compound (published Japanese patent application No. 52-128,903). In this cleaning agent, however, the suppression of the abovementioned excessive dissolution is insufficient, and the cleaning bath is discolored. Furthermore, the tannic acid compound adheres to the can; possibly changing the quality of the contents packed in the can, which is undesirable from the viewpoint of food hygiene and appeal; and which creates difficulties in conveying of the can by automatic processing machinery. Furthermore, the amount of the tannic acid compound consumed in the process is large, which is economically undesirable. A cleaning agent with a specific type of alkaline ingredient and a specific compounding ratio also has been proposed (published Japanese patent application No. 53-102,309), but this composition has insufficient suppression of the above-mentioned excess dissolution. The same may be said of a cleaning agent containing an alkali metal silicate (published Japanese patent application No. 56-158,879). Furthermore, a cleaning agent has been proposed to which a phytate compound is added (published Japanese patent application No. 55-110,784). This cleaning agent has the defects that the phytate compound adheres to the can, obstructing its conveyance by processing machinery, and the consumption of this expensive compound is high, so that it is economically unprofitable.